In recent years, along with the explosively spreading use of the Internet, electronic mail (e-mail) is extensively utilized as one of its applications. In this electronic mail communication, a client apparatus for transmitting mail (transmission client) registers its own address with a mail server apparatus (transmission server) in advance. When sending an e-mail, the transmission client, using the simple mail transfer protocol (SMTP) for instance, sends an e-mail message (mail) to which the mail address of the destination (destination address) is attached, to its own transmission server. Having accepted the mail, the transmission server checks the destination address, and transfers the mail to the mail server of the destination address (reception server). In this manner, the mail is delivered to a mail box in the reception server. The client apparatus receiving the mail (reception client) accesses its own mail box and, using the post office protocol (POP) for instance, retrieves mails stacked up in the mail box. In such electronic mail communication, even if the addressee is not present, mailed information is automatically accumulated in the mail box of the reception server, which is the receiving party. Therefore, e-mail is an excellent means of communication that is not constrained in time or space.
Thus, electronic mail communication via the Internet is extremely convenient as well as useful for diverse purposes, and the number of mail users via the Internet has dramatically increased in recent years, making it a common infrastructure technology in both societal and consumer life aspects. However, where the Internet is used as the communication line for exchanging electronic mail, there arises the grave problem of lack of confidentiality. Thus, when an electronic mail is sent via the Internet, the contents of the mail will be subject to the risk of being overseen, altered or destroyed by a third party. To eliminate this risk, sometimes a leased line may be used that does not traverse the Internet. Unfortunately, this entails a heavy burden of facility management. If connection to the Internet is used in parallel, security with a firewall or the like would be additionally required.
It is also conceivable to introduce a virtual private network (VPN) which would take charge of fully encrypted communication at the physical network level for communication with specific addressees, such as important clients or suppliers. This VPN provides a method of using an open network, such as the Internet, as a private exclusive network, and requires the use of a network combined with VPN-dedicated routers and firewall products, a VPN-compatible protocol and the like. However, this would require all the electronic mail users involved in a specific mode of communication to use the same facilities, inevitably entailing the financial burdens of additional hardware and management and accordingly a lack of common usefulness.
Another method is to encrypt the text of the mail to be sent and the attached file or files and digitally sign the mail every time communication is initiated. However, such an arrangement would presuppose installation of the same software for encrypting, such as a public key, for both the transmission client and the reception client, and accordingly cannot be convenient for common use. The heavy load of encrypting on the computer would also pose a major problem.
Therefore, in the operational aspect, the Internet cannot be used for the exchange of confidential information in some cases, and as a matter of fact traditional mail, communication over switched lines and facsimile are still frequently used instead of electronic mail.
Thus, the conventional electronic mail system using the Internet cannot secure confidentiality by generally applicable arrangements. In view of this problem, it could be concluded that the connection least susceptible to overseeing, alteration or destruction of the communicated contents (secure-free connection) can be achieved by using a switched line, not through the Internet, for connection between mail servers, when sending a highly confidential electronic mail. This secure connection would require (1) that the switched line be connected only when it is being used for electronic mail communication and be fully open at other times, (2) that the other party to be dialed not be limited but the destination of each electronic mail be individually dialed and (3) that the connection be compatible with common operation with another mode of communication service, i.e. facsimile mail.
A conceivable way to meet these requirements is the use of a dial-up router for secure connection. This dial-up router is a device combining a function to access an Internet service provider and a function to connect to a LAN. A dial-up router automatically establishes connection at a request from a computer connected to a LAN for connection to the Internet and automatically cuts it off upon termination of the communication, and can connect a plurality of computers to the Internet at the same time without having to do any special setting. This dial-up router is usually compatible on its public line side with an ISDN line and on its LAN side with 10 Base-T of Ethernet (R). It involves less trouble and cost than to contract for a leased line for connection to a LAN or to connect the computers one by one to the Internet with a telephone line or the like.
However, for a dial-up router, the telephone number which is the destination of communication usually is fixed to a specific provider, and it is impossible to establish direct connection with another party with whom communication is desired from time to time. Its mechanism does not allow recognition of the particulars of intended communication, and cannot be expected to permit connection only during communication, for instance, dialing only at the time of starting mail transmission and cutting off the connection immediately after the end of transmission. Therefore, transmission and reception of an electronic mail by secure connection requires a function to establish connection only during the period of actual communication and ensure safe transmission and reception, such as the function of G3 facsimile (a communication apparatus according to the T.30 specification of the ITU-T Recommendation of the International Telecommunication Union (ITU)).
Most families and small offices use dial-up connections for mail communication. A dial-up connection is a form of connection using a public line for connection to, for instance, the Internet. By this way of connection, a user would enter into a contract with, for example, a service provider and establish connection to the provider with a modem or a terminal adapter (TA) via a subscriber telephone line or the INS network.
Now, in this way of electronic mail communication by dial-up connection, the line fee is charged as long as the line remains connected irrespective of the actual quantity of mail communication. In mail reception, it is a so-called general delivery system, which requires the addressee himself or herself to connect the line to check for and retrieve any delivered mail. Even when prompt processing is needed, such as in accepting an order, the pertinent electronic mail stays in the mail box of the reception server, i.e. in the POP server, which is the deposit station of electronic mail unless the receiver himself or herself actively picks it up. Solution of this problem requires uninterrupted connection, but this entails an extra telephone charge, which an ordinary family or a small business may be unable to afford. For instance, G3 facsimile has a function interlocked with the sender's action to automatically accept a message and cuts off the line connection as soon as the communication of the message ends (straightforward signaling function), but no existing electronic mail transmission/reception system has any such function, and a mechanism to enable the sender to immediately send an electronic mail to equipment ready to hand on the intended receiver's side is needed.
In view of the foregoing needs and limitations, an object of the present invention is to provide an electronic mail system excelling in safety, capable of establishing connection between mail servers, when sending an electronic mail, over a switched line without relying on the Internet.
Another object of the invention is to realize efficient electronic mail transmission/reception capable of reducing the turnaround time required for mail delivery.
Still another object is to realize electronic mail communication between mail servers for direct mail delivery using communication procedures based on a standard protocol, such as the one prescribed in the ITU-T Recommendation T-30.